1. Technical Field
The present disclosure relates to a slot array antenna.
2. Description of the Related Art
An array antenna including a plurality of antenna elements (which may also be referred to “radiating elements”) that are arrayed on a line or a plane has its use in various applications, e.g., radar and communication systems. In order to radiate electromagnetic waves from an array antenna, it is necessary to supply electromagnetic waves (e.g., radio-frequency signal waves) to each antenna element, from a circuit which generates electromagnetic waves (“feed”). Such feed is performed via a waveguide. A waveguide is also used to send electromagnetic waves that are received at the antenna elements to a reception circuit.
Conventionally, feed to an array antenna has often been achieved by using a microstrip line(s). However, in the case where the frequency of an electromagnetic wave to be transmitted or received by an array antenna is a high frequency above 30 gigahertz (GHz), e.g., the millimeter band, a microstrip line will incur a large dielectric loss, thus detracting from the efficiency of the antenna. Therefore, in such a radio frequency region, an alternative waveguide to replace a microstrip line is needed.
It is known that using a hollow waveguide, instead of a microstrip line, to feed each antenna element allows the loss to be reduced even in frequency regions exceeding 30 GHz. A hollow waveguide, also known as a hollow metallic waveguide, is a metal body having a circular or rectangular cross section. In the interior of a hollow waveguide, an electromagnetic field mode which is adapted to the shape and size of the body is created. For this reason, an electromagnetic wave is able to propagate within the body in a certain electromagnetic field mode. Since the body interior is hollow, no dielectric loss problem occurs even if the frequency of the electromagnetic wave to propagate increases. However, by using a hollow waveguide, it is difficult to dispose antenna elements with a high density, because the hollow portion of a hollow waveguide needs to have a width which is equal to or greater than a half wavelength of the electromagnetic wave to be propagated, and furthermore, the body (metal wall) of the hollow waveguide itself also needs to be thick enough.
As waveguide structures to replace microstrip lines and hollow waveguides, Patent Documents 1 to 3, and Non-Patent Documents 1 and 2 disclose structures which guide electromagnetic waves by utilizing an artificial magnetic conductor (AMC) extending on both sides of a ridge-type waveguide.    [Patent Document 1] International Publication No. 2010/050122    [Patent Document 2] the specification of U.S. Pat. No. 8,803,638    [Patent Document 3] European Patent Application Publication No. 1331688    [Non-Patent Document 1] Kirino et al., “A 76 GHz Multi-Layered Phased Array Antenna Using a Non-Metal Contact Metamaterial Waveguide”, IEEE Transaction on Antennas and Propagation, Vol. 60, No. 2, February 2012, pp 840-853    [Non-Patent Document 2] Kildal et al., “Local Metamaterial-Based Waveguides in Gaps Between Parallel Metal Plates”, IEEE Antennas and Wireless Propagation Letters, Vol. 8, 2009, pp 84-87    [Non-Patent Document 3] Tomas Sehm et al., “A High-Gain 58-GHz Box-Horn Array Antenna with Suppressed Grating Lobes”, IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 47, NO. 7, July 1999, pp 1125-1130.